1. Field of the Invention
The present invention pertains to a method for fracturing earth formations to stimulate the production of fluids therefrom by over-pressuring the wellbore with an expansible fluid such as dense phase carbon dioxide or nitrogen and initiating the fracture with perforating guns or release of the fluid into the formation through a frangible closure.
2. Background
In hydraulically fracturing earth formations to stimulate the production of fluids therefrom, a long-standing problem has been the inability to sustain high pressure and high flow rates of the fracturing fluid during fracture initiation or extension. In deviated wells, in particular, inadequate pressure/flow conditions at fracture initiation will produce a near wellbore "kink" in the fracture which will tend to restrict the flow of fluids to or from the wellbore once the fracture has been formed. U.S. Pat. No. 5,074,359, issued Dec. 24, 1991 to Joseph H. Schmidt and assigned to the assignee of the present invention, discusses the problem of improper fracture formation from deviated wells, in particular. The '359 patent is directed to a method for orienting the casing perforations to minimize improperly formed fractures at or near the perforations.
Previous efforts have been made to provide sustained high pressure flow of fracturing fluid into a formation to initiate substantial fractures and sustain fracture growth. U.S. Pat. Nos. 3,170,517 to Graham, et al; 3,200,882 to Allen; 3,393,741 to Huitt, et al; and 4,718,493 to Hill, et al all describe hydraulic fracturing methods where a column of liquid is pressurized and released suddenly to act on the earth formation from a perforated well to improve fracture initiation and extension. The Hill, et al patent suggests mixing the fracturing fluid with a proppant and a pressurized gas. Still further, U.S. Pat. Nos. 5,131,472 to Dees, et al and 5,271,465 to Schmidt, et al describe further improvements in over-pressuring the wellbore prior to fracture initiation or extension.
One problem associated with the methodology described in the above-mentioned patents is that the substantial column or quantity of fracturing liquid, usually water which has been treated with a friction-reducing agent such as guar or HPG, resists the high rate of acceleration required to assure a non-kinked fracture and to enhance fracture link-up between vertically spaced wellbore perforations. By placing a column of liquid, such as water, in the wellbore and the well tubing and pressurizing the well tubing above the water column, a substantial quantity of non-expanding liquid, in the amount of twenty-five to thirty tons in a typical well, must be accelerated into the formation through the wellbore perforations. Accordingly, the initial velocity and kinetic energy of this liquid column is difficult to raise to optimum levels to achieve the type of fracture extension and growth desired. It is to this end that the present invention has been developed with a view to improving the initiation and extension of hydraulic fractures using the so-called over-pressured wellbore fracturing methodology described in various aspects in the above-mentioned patents.